by using change of variable and conjugation by a function, which are the only two automorphisms of \({\mathcal {D}}\) ; we shall always assume \(L\)  to be in standard form, i.e., \(u_1 (x)=0\) . We note that in [1]}, for completeness, the authors recall a(n essentially formal) proof of the facts we mentioned, to bring \(L\)  into standard form.
